Secret signaling



Usi. M, 1928,

J. FARRINGTON SECRET SIGNALING Filed July 2, 1925 3 Shees-Sheet l C', s FM A y. l?)

` 1,687,932 J. F. FARRINGTON SECRET SIGNALING Filed July 2, 1923 :s sr1e=ts-sheet` 2 20o-2000 E Y gof o M .M y F aooorsb socie 7700 ooo 1 +o 90 l To 2oo 2ooo 7500- 90 9500 @mi i @m my ma.

J. F. FARRINGTON SECRET SIGNALING Filed July 2, 1923 3 Sheets-Sheet Patented Oct.. i6, 1928.

TES

,UNITED `sra PATENT4 OFFICE.

JOHN F. FARRINGTON, OF FLUBHING, NEW YORK, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION F NEW YORK.

SECRET SIGN ALING.

Application filed July 2,

The present ii'iyention relates to signaling and has4 |mrti-cularly to do with scouring secrecy in the transmission ot signals.

An object of the invention is to secure a' 5 lilith degree ol secrecy in the transmission of messages by vradio or carrier while still enabling the messages to be received at an authorizcd .station with higfh quality ot rcliroductirm.

it has been euggestcd to secxn'e,sec1eey by tranflmitting only Waves the frequency components of which have a yfrequency order diftercnt from that of the .signal waves. as, l'o:I example, waves, ha ving a 'Frequency order the inverse of that ot the signal wares so that thc lowest frequency component ot" the riginal is tlanz-mitted as the highest ,''lequenrtlf.` and the higher frequency compoucnts ol' the signal are transmitted as successively lower frequency components. Huch a wave is unintelligible and can only be understood by inverting it so as to re .store th'I normal frequency order ot the signal. lt has further been proposed to Cause the inverted ircqucncy wave to undergo a cyclic frequency .'ariation to prevent the message l'rom being intm'ceptcd by uSe ot a local source ol' rmrillatioils ot Such a. 'frequency as to give a beat, wave otI the signaling trcqucrwy and of the normal frequency order. this has been acconmlished by introducingT t'reqluency variations into the lrain-:m'lting carrier 4wave .so thatA the entire' band o't" trcqucnciesI transmitted undergoes cyclic lrcquency variatiom-z. Detection ot" such a ware yields the modulation component diri-ctlv. and it' thc cyclic frequency Variations aril not ,Lio rapid as, to produce an audible sound upon detection. no interfer-4 cnce in the detected 4signal results. The dC- tectcd ware insucli :e system is incapable of hein;r undcrstoml since. it represents the inverled 'frcqucncics ot' the signal. and thix .-;\'stcm pour` sses a high degree ol secrecy, therefore. .since thc operation ol' simple detection must be followed b v a frequency inrcrtin;r operation in order. to reproduce the signal. Aa .stated above, the cyclic frequency variations ol' the transmitted waves mal e it impossible I'or one to receive the signal by a single step ol detecting and inverting by use of a local source of beating waves.

1923. Serial No. 648,887.

frequency of the signal component with re- .spcctto the carrier component. The carrier component may have a cons 'ant or -a variable frequency. and 1 n either case the transmitted wave components. representing the signal,

commonly called the side, band components, hare cyclic frequency variations entirely independent of the carrier. -Upon simple detection ot' such a wave, a band of audio frequency con'iponents iS obtained, which hand, as a whole, undergoes cyclic frequency variations. it the carrier wave has been modulated by speech of normal frequency order ybut varying in frequency as to the entire speech band, a Similar speech hand shitting upward and downward in the frequency spectrum is received upon simple detection, and such a wave, is not only incapable of being understood as it is but is very diticult to transform into understandable speech, i

since in order to transform it', continuous oscillations ot' the proper absolute frequency and with the. same frequency variations as the detected band are required. By inverting the speech at the transmitter, a still greaterdegree ot unintelligibility is produced in the detected wave. i i

The signals sent according to the invention are received at an authorized station by combining with the detected audio band of varying frequency a fave otI similarly varying- 'frequency and of proper absolute tr"- quency, and this wave is obtained" at the receiver by operating upon Ta control wave conmoncnt sent from the transmitting stir, tion and reproduced at the receiver upon simple ,lctection. rI`his control component, however. may be ot the same amplitude as the signal components and may have a frequency lying in the normal Signalvtrcquency rance. so that an outsider would not have any reason to consider the control wave component als anvtlxing but one ot' the signal conmoncnts. torcover. in case it should bc learned that a certain component of the dctccted wave is a control component, this information ,would not enable the signal to be reproduced since'the wave that is essential to the reproduction of the signal-is, according to the invention, onlyderived by QP'- between the two stations, assuming both s t a necessary to reproduce wave by t 'single component transmitted from the transmitting'station, a plurality of waves may be transmitted either from onestation, or, preferably, one from each of a plurality of stations, as, for example, one from each of two communicating stations, and at each station the heat between the transmitting waves may be used as the control component. If, for example, Chicago were communicating with Seattle and at each station the control Wave were derived as the beat between two Waves, one sent from eachV station, the only location-at .which these components would appear ofi-equal intensity would be half way tions had equal power, of course. At a point between the stations but not equi-distant from them, one of the components would be stronger than the other, so that an outsider who might be looking for a control vcomponent would be led to sus ect that 4the stronger component would be t ie key. Similarly, at New York for example, any person who could receive Chcag but' who could not receive Seattle would loe-unable to hear evenv one side of the conversation since. he could not receive the component from -Seattlle the control coniponent. r L L' The invention may be briefiy and specifically characterized, therefore, as fundamentally eomprisingthe transmission of a' carrier Wave with'a side band gf quency relative to the carrier frequency. Preferably the speech to be'transinitted is invertedb modulating a variable frequency he speech and selectingl the lower l Aside band. The variable frequency Wave used for inverting is derived from a control Wave, which, together with the inverted speech, modulates a carrier wave for transmission. At the receiver, the Wares are detected and the control component is used to reproduce an inverting wave identical with that used at the transmitter. This inverting wave is then employed to re-invcrt the detected band and reproduce itA as understandable speech.

The various features and objects of the invention will be more fully set forth in thc followingr detailed description, in coijuuzction with thc accompanyingr drawings. ln the drawings, Fig. l is a schematic circuit arrangement of one terminal station ol a system accordin,r fiolhc invention and Fig. 2 is asimilar showing-of a slation adapted to cooperate with the station of Fig. 1. Figs. 3, 4 and 5 are modiliod circuit arrangements which may be culploydd in lhc system shown in Figs. l and 2. Fig. showsa two-way Station with a somewhat 4dill'erent arrangevaryjaig fre! ment. Fig. 7 shows a two-way station employing modifications similar to those shown 1n Figs. Il and 5; vand Fig. 8 shows a modification of the arrangement of Fig. 7.

Referring first to Fig. 1, the transmitting side will be seen to comprise a speech inverting portion comprising the microphone circuit 1, a modulator M,L and the source 2 of inverting frmequencies; and the high frequency transmitting portion comprising the transmitting modulator M2 and the carrier wave source 3. The modulator Ml is preferably of the balanced tube type shown, for example, in the Hartley Patent 1,419,562

June 13, 1922, and the input'circuit is sup- 50 plied with continuous waves from the amplifier A, and With'speech waves from themicrophone. circuit'. 'l'. rPhe source 2' is'. preferably* l a ',if'acuumtnbe 'oscillator ef thetvpe shown.

in the Hartley Patent 1,356,763, ctober 2G, 9"

ings to indicate that the mean frequency or Q5 l band of frequencies written before the symbol varies between upper and lower limits.` differing from the mean by the amount written after the symbol. v

of the source 2 may. convenl Thevarying frequency wave from source 10" 2 1s impressed on' the harmonic generator HG, and the sixth harmonic is selected by the filterFl from the harmonic generator upd applied to the amplifier Af. The continhous Wave supplied to the modulator M, ds, 10

therefore, a wave of varying frequency having the extrenu. frequencies 2964 and 3036.

The harmonic generator may be of the type disclosed in Kendall Patent.1,4l1l6,7'2. February 27, 1923. The filter F, uniform frequency transmission charactcristic over the range of variation of 'the transmitted ware. This result may" he accomplished'by constructing the filter l", yas a band filter in thc manner described in (he H5 Campbell Patent 1,227,113, May 22, lill?. Each of the other filters shown tl'irougl'mut the drawing may hc constriutcd in the maziner disclosed yin thc Campbell patent and should; have a W may be dcsigncdto have the transmission lf3" range required in any particular imztancc.

Thefilter F., iu thc speech circuit is preferably dcsigncd to transmit lhc essential speech frequencies. -li'or example 2ll0to 200() cyclos inclusive. is ti'a'nsn'iitlcd through the hand filter l anda amplifier A., lo the modulalor'M2 l'rom which thr outgoing` wave is supplied through the radio frequency amplilicr A, to the antenna (S.` Thefsource of carrier waves il The output ot' thc modulator Pri, 12

play hc |30,

ill)

of continuously varying frequency to the har.

monifgenerator liti, from which, as stated, the wave ot' ititit) J. it cycles iS selected hy the tilter l amplified at A, and applied to the modulator Ml. Also, the essential speech t'rcquencies are selectively transmitted to the lilter 1"? and supplied to the modulator M,. By the. well lmown action of the modulator, two side. hands are produced` each havingy a frequency width of 1,8()0-cyclcs at any instant'. When the inverting;r frequency is 55000 cycles, one of these side hands extends from ltltlt to 28H() cycles and the` other extends from i320() to .'itltlt) cycles. The two side hands and the, inverting frequency vary together upward and downward in frequency through a total. variation range ot' 7i? cycles. rlhe filter l", is a hand tiltel' designed to transmit. selectively only the lower side hand from the modulator M,. By referring to the lilartley latent 1,419,562 mentioned ahove. it will he seen that a property of the modulator M., is to suppress the unmmlu-lated component of the applied invertingv wave. and also ot the applied speech waves; Even though the frequency range of the filter l",JL overlaps that of the tilter l". none olf thev origiihily speech can pass through to the carrier modul,`

lat or M2 on account of the suppressinf.r action ot the modulatin circuit M',. 'l'he lower side hand transmitted through the filter FI., represents inverted speech, that is, the lowest speech trequcncy has heen transformed into the highest frequency ot the hand and the highest speech frequency has heen changed to the lowest frequency otl the. hand. Also, the hand has heen shifted upward in t'requency a slight amount, since` as stated ahove, it now extends Atroin ltitlti cycles to 280() cycles with a cyclic shit't in frequency to the values 36 cycles higher and t cycles lower than these limits.

'lhe inverted frequcln ies modulate the radio waves from the source fi in the modulator M., and the resulting; modulated wave is transmitted trom the antenna (3. ltis not necessary that, hoth side hands ol the radio 'frequency wave. he transmitted hut it is sufficient it' only a single side hand of the inverted speech and control wave is trans.- 'initted Accordingly. if desired, the. modu lator M., may/he of the. same type as modulator M, or` 'ot the type shown in the (arson Patent 1.348.306 .lune l5, 1920, Vso as to prevent transmission of the unmodulated radio frequeneywvave, and the antenna 6 may he tuned to select one of the two side hands t'or transmission.

lt' the wave so transmitted is intercepted on a simple detecting receiver w tether lone or hoth side. hands are transmitted` and whether or not the carrier is suppressed` only waves incapahlc ot hcing understood will he received. l"or the most that can he ohtained 'i from a single stage of detection will he a hand ol' waves of speech frequency width continuously varying 1n frequency and shiftled from the normal speech frequency wave.

It' this hand occupied the normal speech range it would he unintelligihle on account ot this c vclic variation. 'lo render the wave. intelligihle..therefore, the frequency variations must hc neutralizedand this is practicaly impossihle without; a knowledge ot the manner and `t'requem y ot' the variation.

The waves so transmitted may he. received and reproduced as understandable speech hy a station ot' the type shown in` Vig. 2. The waves intercepted on the antenna H are detected at l), and the. resultingl low frequency components coniprif-ting in part. the control wave ot' fait) 1": t3 cycles and the inverted speech waves4 are separated hy the respective filters l". and l".. "lhe control waves are applied to the harmonic generator lltz, which may he identical with ll(i and the filler if(l selectivelytransmits the six'th harmonic ol' 30H() :iF A, and liltcr F, connected to the denmdnlatw l).\"l,. 'l`hc inverted speci-h waves are sch-ctively transmitted through the tiltcr Fm am` pliticr A5 and preterahly through a second filter l" and alnpliticr to the demodulator1 l)l\/l i 'lhe construction and mode ol' operation ot the deniodulator DM, may he identical with that descrihed ot the n-nnlulatoili/l., its purpose heimr to re-invcrt the received inverted speech waves hy comhining them 'ith the same harmonic ot the control wave as was u'sed in the. transmitter. 'lhe lower-'side hand produced in the dcmodulator DM, will extend from 2li() to Elf/'titl cycles and will have'the same order of frequencies as the waves in the output o'f the tilter F., at th'e transmitting station. This lowersidev hand is selected hy the filter Ik, which may` -he identical. with the filter IQ, and is applied to fit', cycles to the amplifier the receivers l0 where it is heard as u n.'ler-v i ,standahle speech'.

ln a`.\-twoway system there will heat onen terminal, a transmittingr station of the type shown in"-Fi; l, and at the other station a receiving circuit ot the type shown'iu Fig'. 2. lt. is not necessary, lnnvever, to duplicate.

these eirc.nits,'in order to provide a transf of that terminal, thus avoiding duplication of the harmonic generator circuits. Similarly in Fig. l; the inverting deinodulator DMQ, may be supplied with an' inverting wave from the output of filter F1. The remaining elements of the respective Itransmitter and -receiver'may be identified from the reference characters; which are the same Y as are employed in the other respective fig- Y Waves of 700 cycles. and either or both of I Amay be derii-'ed at the l the limits of 7500 and per side hand comprisingi` frequencies from hc source 2.

these frequencies may be varied by connecting the appropriate tuning condenser 21 or 22 or both of them to the motor 23. The .heat frequency wave between the two waves tlnis produced will have a frequency of the orller of 500 cycles. This beat frequency.

transmitter by applying the waves to the detector Dg so that a variable wave of the order of 500 cycles is produced in its output circuit. Both of the waves from thesources l9`and 20 may he vapplied to the outgoing modulator so that they are sent to the distant station. The cir'- cuit arrangement of Fig. 3 may be substituted for that portion of Fig. l shown to the left and above the dotted line A-A. A t thereceiving station the products of deteetion-of the transmitted wave will include the beat waveV between the 200 and 700 cycle modulationl components at the transmitting station.

A modified system for producing the inverted speech frequency waves is indicatedin Fig. 4, which arrangement may be substituted for that portion of the system of Fig. l shown to the lel't of the dotted line B-MB. In the modified arrangement. the speech waves from the filter F:I modulate at M4, a varying frequency wave ofthe order of 7500 cycles, supplied through the filter F10 from thc harmonic `generator HG1 associated with The 7500 cycle wave is obta ined as the 15th harmonic and the Frequency ot' this wave will lic at anyinstant between 7410 cycles. -The up- 7700 to 0500V cycles cycles upward and with variation of 90 downward from these ruines. is selected by the filter F11 and ap-` .plied lo the modulator M5. The filter F@ selects the 21st harmonie from the harmonic generator HG1. This harmonic having a l'rrquclu'y of '1').5 "l-\:L]20^cycles is applied to the modulator M5. The filter Flb selects the order. The modulators BL lower side band of the modulated wave from modulator M5 This side band extends from 1000 to 2800 cycles, the limits of which. vary cyclically through 36 cycles upward and downward from these values. The advan. tage'of this type of inverting arrangement is that. the inverted wave does not depend upon any balancedcircuit arrangement to prevent the occurrence in the output circuit of the modulating system of speech frequency comp onents having the normal speech frequency and M5 may he single tube modulatorsfsince thereis no possibility for speech to pass directly from the filter F2 through the filter F13. Y

Thether or lnot the modification shown in. Fig. 4 is used at the transmitting station, it may be desirable, at times, to use at the rc-4 ceiving station the type of inverting system shown in Fig. 5, this system being suhstituted for that portion of the circuit in Fig. 2

to the right of the dotted lineG-G The operation of the circuit lof Fig.. l5 will he clear from the description already given of Fig. 4. Tl v ware ot' 500:*:6 cycles isx applied to the harmonic generator HGr2 from which the lilters F14 and F15 derive, respectively, thev frcqueu` cies 10,5001-126 and `7500i90. These' frequencies are supplied through the respective `amplifiers to thel respective modulators'llT and Ma. The detected inverted-speech waves from the filter F5 are applied to the modulator' M(i and the upper side band of frequencies from `S50= to 10,300'with their cyclic variation of 126 cycles is selected by the filter Fw'and applied to'thc modulator ie receii'edfand detected control M7. The'ilterA F17 selects the lower side I band from the modulator M7', this side band representingr normal speech liietwcen the frequencies 200 and 2000 cycles. of Fig. 4. the circuit of Fig. 5 removes the possibility of the received speech being confused or distorted'd'ue to the direct trans-l mission to the receiversl'O of components .from the detector in case of an unbalance in los' As in the case the suppressing circuit DMl when such a 1 circuit is used.

l'nthe system showninFig. 0, the control of the inverting frequencies .at the trans` mitting and receiving'stations is effected by using two Waves and deriving their beatwave at both stat-ions. The system of. this figure, however, diff ers from thatof Fig. Sin that one of the two components is transmitted from one of the two commuuicating stations I and the other component is transmittedfrom y the other station, ln this figurait willbe assumed that a variablefrequency' ofthe order of. 700 cycles is contiiulouslytransmitted from thc-antenna 25 as a' modulation ot' the carrier 'wave from source 2G in tli` mfululator-lvlH hy the' variable frcquenqtt `Where they conihinc ponents would not appear vetted speech Waves resulting from modula- At the distant cmiperating station, a source similar'v to 2T is assun'ied to be source emitting a wave of resent, this modulates the` carrier 2U() cycles which 1wave at that station for trai isinission to the station in Fig. t5. 'lhis 2U() cycle` wave may have either a varyingy or a constant fri qiirne'v, but: for simplicity, in the prei-vent case it will lic assumed to ,have a constant frequency, ill its frequency, were cyelirally varied, the total range of variation of the signal as finally trai'ismitted could be increased. The wave transmitted is received on the antenna and detected at l), to yield as one of the conq'ionents a :200 cycle wave which :selected by `the filter FW 'l`his 2U() cycle wave is supplied to the detector l)1 togthcr with a wave fron'i thel source 2T and the heat note olf 50() cycles :Si: f3 is .selected by filter' Fvg, :ind applied to the `liaiflioliic generator flfir., 'lille filter lzr, selects the sixth harmonie or fino() i: 556 and this is 'used as the inverting frequencyv for the modulator 5L, and the deinodulator DMJ. The distant terminal station may be a duplicate of that of Fig, (3 except that as stated, the source 2T of' that station has a frequency of 200, and the filter l?38 `of that statici. .selectivil of T0() f3 cycles. rl`lie inverti-d speech frequencies occupying the range lofi() to lwifi cycles with a plus and minus iti cycle vari ation' received from the dis tant station on the antenna 255 and detected at l), are selectively transmitted through the filter lfm to the deinodulating circuit DM2 with the inverting;l wave from the lilter Fm to yield understandable speech, for application to the receivers 3f).

The systei'n described in Fig. f3 possesses a greater degree of secrecy than that described in theprcvious figures for the reason that necesi-ary to receive the control comA poncnt from ear-h of the two intercommunieatingl stations in order to receive even one side of the conversation.V Thus, if an outsider should have a directional receiver and should arrange it to receive from only one of the two stations he would not', obtain sufii cient wave components to enable him to rc1 produce understandable speech. Furthermore, at any point except midway between the tivo stations, on the assumption that they transmit equal power, the two control comto be equal and an cx ierimetcr would he unlikely to assume `that the two c-'iioponei'its of unequal magnitude and of' different frequency were an essential part of kthe secrecy scheme. A station so situated as to be able to receive only one of the two intercommunicating stations as for example, a receiver at New York which could but not Seattle, would not be allie to pick up even'one side of a conversation between C iicago and Seattle sinceit wouldy be unable to receive the control component sent out from Seattle, Which is essential to the reception ot' any of the message Wai/9S.

Fig. l

receive Chicago A illustrates a system similar to that shonn in Fig. t3 but provided with the double modulation ii'ivcrtineY arrangements of `the type' described ii'i connection with Figs, 4 and ln Fig. T, the speech from the microphone circuiti modulates at Mp, a frequency of o() t iltiderived through the filter Fu. ,from the harmonie generator HGM. The filter transmits the upper side band or 770() to fiilili with a plus and minus {it} cycle variation in the side hand frequenciesan`d this band is applied to the modulator Mu. The modulater M:I supplied with a wave of 10,500 i126 derived through the filter F24 from the harmonic generator Htl4 and the resulting lower side hand from the modulatorl Mu is selectively transmitted by the filter F25 t0 the transiniting modulator Mg, the frequency ot this side band beingl to 2800 with a band variation of plus and minus 36 cycles. 'l'his side hand represents inverted speech of varying. frequency. A wave from the source 2T to TUO cycles i 6 also modulates the outghlllg (IlIl'lQI'. Y

"l'he `received Waves incidenten the receiving .antenna 28 contain a 20G-cycle control compoiient produced at the distant station as explained in connection with Fig. 6,

and this f :omi'ionent resulting from detection ot the received waves vat D., is transmitted through the filter FW to the detector D4 where it combined with some of the 70o z; (L cycle wave from the source 27. The heat wave-ol' 50i) v f3 cycles is selected by the filter ll and applied to 'the harmonic generator f'lfiq. 'lhc inverted speech coniponeiits of the received `vave are selecte by the filterlQ, and applied to the modulatorf\1` where they modulate thel0,5()0 I 326 cycle wave.

lower .side band and applies it to the modulator M1.; where it modulates with/some of the T50() fr; il() cycle wave. The filter' F2X selects the loner side band or normal speech and applies it to the receivers fil).

'l'h'e invention lends itself readily to various schemes of frequency transformations and it is to he understood that the numerical valut-s thus far given are to be taken merely as examples and not as in any wise limiting the frequencies that maj-C actually ie used. A further ,sf-heme enq'iloying somt-.what different frequencies from those above given, is shown in Fig. 8. 'l`he general arrangement of Fie'. 8 is identical with that shown in TfeJ-:cept for the portion 'of ythe circuit included lin the dotted line enclosure of the two figures. The filters o f Fig. 8,- however, have The filter F2, selects the The modulators M11 and MQ, are,

di'erent frequency ranges as indicated on the drawing. The elements in Fig. 8, similar to those in Fi 7 except for the frequency ranges, bear tie same reference characters witli a. prime added. The result' of ymaking the substitution' of the circuit of Fig. 8 for the circuit of 7 is that the control wavev component genery .tion except that the control component generated at that station is now 3100 cycles. this 3100 cycles component may be varied in frequency or not, but it will be assumed, in thev present instance, that it has a constant frequencyg Y Y In the operationV of Vthe modified circuit of Fig. 8 the control Wave component from the source 27' is suppliedl to the harmonic generator HG, and the 4th harmonic, thatis, 10,000 i 36 cycles is now selected by the filter' F 24, this filter being designed in the modi; lied arrangement to select these frequencies. therefore, both supplied with the 10,000 36 cycle frequency. At the distant station the control component is, as stated, 3100 cycles constant frequency and this modulates the outgoing carrier in the same way as described in connection with the previous figures. The wave received at the station in Fig. 8 upon detection yields a constant frequency component of 3100 cycles which, it will be as sinned, is selectively.transmitted by the filter FQ., and applied to the harmonic generator HGW The 4th harmonic or 12,400 cycles is selectively transmitted bythe filter 1*",2 and is supplied to the two modulators Mfw and M Speech currents from the microphone filter are impressed on the modulatorM1o where they modulate the 12,400 cycle frequency and the filter F23 is so proportioned as to transmit the lower side band or the band yof frequenciesl 10,400 to ,12,200 cycles. yThisband is supplied to the ,modulator MQ, where it modulates the varying frequency of 10,000

i 36 cycles and the filter Fg, selects the lower side band which it will be seen com-` prises frequencies from 400 to 2200 cycles aving a frequency variation of plus 4and minus 36 cycles. As stated, the 2500 2*: 9 control com onent together with the inverted varia le frequency speech waves are applied to the transmitting modulator M. In this modification, therefore, the inverte( voice frequency'waves are shifted only a very small amount from their normal frequency range and the controlcomponent has a frequencyhigher than any ofthe signaling components. l I

The received inverted speech frequency components from the distant station comprising the band 400 to 2200 cycles with a fre,- quency variation of plus and minus 36 cycles is selectively passed through the `filter F421' and applied to the modulator Mltwherc it is Irom the examples which have been given it will be clear that a greatvarety'of fre-- queney values may be employed in carrying out th'eprinciples of the invention;

Except in Figs. 1 to 5 no amplifiers fhave been Vindicated but, it will bc obvious to provide amplification at any desired points.

in the system.

In some instances, it has been assumed `that a harmonic generator has supplied a vharmonic frequency of a high order'such as tlie 15th, 21st,.'cte. In practice, it may be found Vdesirable to employ a number o'f harmonie generators in tandem when harmonics of thisV order are to be developed. For example, a

harmonic producer capable of efliciently producing harmonics up to the 5th may supply.

these harmonics to a `second harmonic generator capable of producing harmonics 11p-to say the 4th or 5th` and this process may be continued as may be desired. "Vherever 'a sngle harl'nonic generator is shown, therefore, it is to be assumed that it'may be re placed by anyrequired number offharrnonic generators.

las

For convenience in illustration ithas been'4 l assumed that the carrier transmission was 1n y the form of a radiated wave and also thatthe speech transmitter and receiyer were di# rectly associated wfth the radio transmitter and radio receiver, respectively.' It is to be understood, however, that the invention is applicable'to wire transmission and thataA two-Way telephone rline may be arranged .in any known manner for two-way repeating into the Carrier system.

lVliat is cla'med is: 1.7The met lod of secret signalinfr comprisingmodulating a carrier wave tby the signal'to be transmitted to produce side bands of signaling-frequency width, and

causing the side band frequencies to vary cyclically'in,frequency with respect to the carrier frequency.

2. The method of secret tclephrmy com` prising transforming a'spcech wave into a wave of eyclically varying frequency, and modulating a carrier Wave thereby.v

3. The method ofsecret. telephony com prising inverting the frequency order of speech waves, cyclically varying the frequency of the inverted waves, and modulating a carr'er wave hy the inverted varying frequency waves.

4. 'lhe method ol' secret telephony comprising modulating a carrier wave by speech waves and causing the side-hand frequencies of the resulting modulatfed wave to vary cyclically relative to the carrier frequency.

'the method of secret telephonycomprisiug inverting the frequency order of speech waves, modulatinga carrier wavehy the inverted vaves, and causingv thefrequencyl of th.: s (le-hand components of thc resulting modulated wave to vary cyclically with respect to the carrier frequency.

'(3. The method of secret signaling comprising inverting the frequency-order ot the I'ncssage or other wave to he transmitted, hy modulat'ng a ywave of varying frequency thel'chyand selecting the lower :side-hand of the modulated wave to the the other components, and modulating a carrier wave hy the selected side-hand components.

7. The method of secret signaling comprising inverting the frequency order of the mcssage Aor other wave to he transmitted, pro ducing cyclic frequency variations in the inverted-frequency wave, modulat'ng a carrier wave hy the varying-frequency wave and hy a control wave, transmitting the wave so modulated to a receiving point, and there utilizing the control wave component to rcproduce the signal wave.r

8. The method of secret signaling compri-sing inverting the frequency order-of the message or other wave to he transmitted, hy modulating a varyinir-frequency wave hy the message wave, moduatfng a carrier wave hy the lower side hand of the ,n'icssage-niodulated varying-frequency wave and hy a control wave, transmitting the modulated carrier wave and utilizing at a receiving'point the control wave component to reproduce a varyirig-frequency wave for dcmodulatini,l the inverted wave to restore the message wave.

f). 'l'he method of secret signaling hetween two separated stations comprising transmitt'in'r from each station `a carrier wave modulated hy a different frequency control wave,

utilizing hoth control-wave components at each station to o'ntain a wave'having a frcqueney-ordcr the inverse of that if'the signals to he transmitted, and modulating thc carrier wave sent from the respective station 'in accordance wth the inverted wave produced at that station.

10, 'I`he method of secret telephony hetween two separated stations comprising transmitting from cach station a carrier wave modulated hy a different frequency control wave, utilizing both control-wave exclusion of.

components at cach station to ohtain a Wave having a frequency-order the inverse of that',l

V l1. ln a s-cre-t signaling system, means to produce a gnaling wave having a cyclically varying frequency, means to modulate a carrier wave hy the signaling waveof Varying f requeney;y means at'a receiver to detect the modulated carrier wave to yield a'varyingfrequency signaling wave, means to produce another vaiving frequency wave, means to modulate 'said other varying frequency wave hy the detected varying-frequency signalin wave to yield the normal signal Wave, an

' means for synchronizing the frequency variwave that is modulated at the receiver.

l2. In a secret ysignaling system, two separated stations, means to provide at each station a wave ol' varying frequency, means t0 modulate the wave of varying frequency. at

ation in the signaling wave with those of the one station in accordance with signaling waves to he transmitted, means to modulate a carrier wave in accordance with the modulated wave ot' varying frequency, means at the other station to detect the modulated carrier wave to yield the modulated varyingfrequcncy tec-ted wave with the varying frequency wave .at said other station to yield the normal signal, and means to synchronize the Waves provided at the two stations and the frcquency variations in saidwaves.

13. In a secret telephone system, means to produce a waveof speech frequencies, means to produce therefrom awave of cyclicallyvarying frequency having a frequency-order the inverse of said Wave of speech frequencies, means to produce a carrier wave, means to modulate said carrier wave :by said cyclically-varying frequency wave, means to rcceive and detect the modulated carrierivave,

wave. means to combine the dea source of Waves of varying frequency,

means to comhine the detected Wave with the waves of varying frequency to reproduce the speech, and means to control ythe frequency and the frequency variations of said source ol wa ves to make the reproduced speech continuously intelligible.

14. In a signaling local sources of waves at cach station,

waves produced from a local source at one station being of different characteristic from those of th e other station, and means at each station controlled hy the local waves'prof duced at the other station for transmitting a signal to the other station.

system, vtwo stations,l

the j l5. In a telephone system, two stations7 1w 4local sources of'lwayes ofrespectivelydiifer- -ent characteristicsfat each of the respective Ystations forcontrolling the transmission of speech waves to the other station, at least odically varyingv frequency,

oneof said sources the Wave 'produced by one -of the local sources at one sta tion differing in characteristic from; the waves produced by the localsources at the other station, and means at each station controllcd ,by `the Wave produccrlnt the other stationdiiering in characteristic from the Waves producedV at the first station for receiving speech.

roducino Waves of ri- P s 16. In a signaling system, two stations, 1ocal sources of waves of respectively different frequency at the respective stations, at least one of said sources producing Waves of periodically varying frequency, means for utilizing said local sources'to produceside band waves of periodically varylng frequency rcpresenting signals, and means com rising u source'of carrier waves of constant iiequency for'trensmitting said side vband Waves bctween seid stations. 1

In Witness whereof, I hereunto subscribe my name this 28th' day of June A. D., 1923.4

JOHNF. FARRINGTON. 

